Such a circuit arrangement is known, for example, from DE 19513441 C5, wherein the two high-voltage sources therein are formed by two amplifier branches, which are respectively provided with a switched-mode power supply, a high-voltage transformer and a rectifier circuit. Each amplifier branch is connected on the input side to a (further) rectifier circuit, which generates a d.c. voltage from a line voltage. Such a circuit arrangement can be used, for example, for the purpose of measuring the loss factor of high-voltage and medium-voltage cables and other electrical components, wherein the loss factor (tan (δ)) of the test object is determined by supplying a test voltage that has a value in the kV range, is usually sinusoidal and varies at low frequency (preferably in the range of 0.01-0.1 Hz) at the test object in question, then determining and evaluating the phase position of the test voltage and the test current induced hereby. “Loss” in the present context refers to the energy that is lost electrically and electromagnetically in the test object and, for example, is converted to heat. The loss factor is a measure of this loss.
Hereinafter a capacitor connected to a voltage source with sinusoidal voltage variation is considered for more precise description of the principle underlying the loss-factor measurement. As is known, a phase shift φ between test voltage and the test current induced hereby and causing charging and discharging of the capacitor develops in such a capacitor. An ideal capacitor, which exhibits no losses of any kind, causes a phase shift between voltage and current of φ=90° for a sinusoidal test voltage. If losses now occur in the capacitor, the phase shift φ between voltage and current is no longer exactly 90° but instead differs from this ideal value by the loss angle δ=(90°−φ). The loss factor to be determined for a test object during loss-factor measurement is now defined as tan (δ), and it permits—in case of a deviation from the values to be realistically expected—an estimate of the degree of possible damage of the test object or as to whether the test object can still be used for the needed purpose.
For high-voltage and medium-voltage cables laid in the floor with XLPE insulation (XLPE=cross-linked polyethylene), which are also exposed, for example, to aging effects, such as the “water-tree” effect, for example, the common test criteria specify, for example, that their loss factor (tan δ) is not permitted to exceed a value of approximately 1*10−3 (see, for example, Cable Manual, 8th Edition, Mario Kliesch and Dr. Frank Merschel, Ed. Rolf Rüdiger Cichowski, E W Medien and Kongresse GmbH).
Test instruments with a measuring circuit for loss-factor measurement conventionally rely on suitable means for time-resolved measurement of the test voltage and test current as well as on a subsequent mathematical evaluation of the measured data obtained during measurement of the test voltage and test current, wherein the wavelength of test voltage and test current is first determined by means of a (discrete) Fourier transformation and then—using common and suitable approximation algorithms—the more or less exact phase position of current and voltage curves and from this the respective phase shift as well as the loss factor are determined. To achieve a measurement accuracy that is as high as possible or that meets the requirement for the specific application situation, it is of critical importance to determine the phase position of test voltage and test current as exactly as possible. If the capacitance of the test object is very low, the amplitude of the test current will also be correspondingly low, and therefore a test instrument designed to measure the loss factor will be subject to a restriction—depending on the capacitance of the test object and on the selected test-voltage amplitude—of the measurement range if the loss factor is to be determined with the specified measurement accuracy. This restriction of the measurement range is due to the fact that, at very low amplitudes of the test current, exact determination of its phase position with the necessary measurement accuracy is no longer possible.
By means of the test instruments known at present from the prior art, which are provided with a circuit arrangement of the type mentioned in the introduction together with integrated measuring and evaluating electronics for determining the loss factor, the loss factor (tan δ) of a test object can be determined—if the test object has a capacitance of greater than or equal to 15 nF—with a measurement accuracy of approximately +/−1*10−4 by using conventional measuring voltages in the range of approximately 3 kV to 20 kV (rms voltage), whereas the loss factor can no longer be determined with the said accuracy if the capacitances are lower.
To generate the preferably sinusoidal test voltage of low frequency in the kV range, as is often also employed for VLF (very low frequency) cable testing, various approaches have already been suggested in the prior art.
In a first proposed variant for generation of a test voltage of low frequency (see S. J. Kearly, R. R. MacKinlay: “Discharge measurements in cables using a solid state 30 kV bipolar low frequency generator”, Fifth International Conference on Dielectric Materials, Measurements and Applications, 1988, pp. 171-174), a voltage source is used to supply a d.c. voltage of +/−30 kV, with which the object to be tested is charged and discharged in well-defined manner by using a high-voltage switch arrangement functioning as a “controllable current source.” Since a d.c. voltage on the order of magnitude of +/−30 kV is always present at the inputs of the high-voltage switch arrangement, which is formed as a cascade circuit and is subjected to closed-loop control as a function of the test voltage, the said circuit arrangement is associated with not inconsiderable electrical losses and corresponding heat development, and so it is impossible or hardly possible to use it in a testing and diagnostic instrument that is as compact as possible while containing integrated measuring and evaluation electronics for loss-factor measurement.
A further switch arrangement for generating a test voltage for the purpose of use relevant in the present case, as was already mentioned in the introduction, is known from DE 19513441 C5. Therein it was decided not to supply an unregulated d.c. voltage for the purpose of providing a circuit arrangement with improved efficiency and for generating test voltages of various and preprogrammed curve shapes. Instead, the test voltage is generated within the meaning of the class corresponding to the invention, by the fact that firstly a d.c. voltage (of variable amplitude) is generated from the line voltage by means of a rectifier circuit (to be connected to the external power network) after which it is transformed—in two amplifier branches functioning as high-voltage sources, each with a switched-mode power supply, a high-voltage transformer and a further rectifier circuit—respectively to a high voltage of variable amplitude, with which the test object can be charged and discharged in well-defined manner via an electronic high-voltage switch or a high-voltage switch arrangement subjected to closed-loop control as a function of the voltage at the test object. The first of these amplifier branches is used to supply a high voltage of positive sign at its output, whereas a high voltage of negative sign is generated at the output of the second amplifier branch.
Besides the features mentioned in the foregoing and also corresponding to the class of the present invention, it is further provided that, by means of the closed-loop controller acting on the high-voltage switch or on the high-voltage switch arrangement, an action is also exerted simultaneously, as a function of the (test) voltage measured at the test object by means of a voltage divider, on the two switched-mode power supplies, so that the high voltage functioning, as it were, as the envelope curve for the instantaneous test voltage at the outputs of the two amplifier branches (or high-voltage sources) is varied as a function of the test voltage for the purpose of minimizing the loss power and/or of establishing various desired curve shapes. With this circuit arrangement is was possible to reduce the loss power significantly compared with the prior art explained hereinabove, thus permitting the use of circuit arrangements of the class corresponding to the invention in compact test instruments with integrated loss-factor determination. As will be explained in even more detail hereinafter, however, and was first discovered within the scope of the present invention, the action on the switched-mode power supplies as provided according to DE 19513441 C5 generates undesired harmonics in the test voltage, and especially in the test current, thus imposing a limitation (which is distinct and, compared with the other interference sources of such an instrument is not inconsiderable) on the measurement accuracy in a test instrument designed for loss-factor measurement.
Further circuit arrangements and methods for generating a test voltage as well as test instruments equipped herewith are known from the undated company brochure entitled “Mobile Testing and Diagnostics: frida with TD, PD-Portable”, Sulz, Austria, pp. 1-37 of Baur Prüf- and Messtechnik GmbH as well as from the data sheet dated 15 Mar. 2013 and entitled “Baur High-Voltage Testing and Diagnostics Instrument—frida, frida TD”, Sulz, Austria, pp. 1-2 of Baur Prüf- and Messtechnik GmbH. Furthermore, the article of CAO, Zhiyu [et al.] entitled “Modeling and Control Design for a Very-Low-Frequency High-Voltage Test System”, IEEE transactions on power electronics, Vol. 25, No. 4, April 2010, pp. 1068-1077 also describes a circuit arrangement of the class corresponding to the invention and a method for generating a VLF test voltage, wherein the test current to be generated therein is adjusted on the output side of the circuit arrangement under feedback control to the “frequency f” and the clock ratio (“duty cycle d”) of a pulse generator, which in turn acts on the high-voltage sources (“power supply block”) that generates the test voltage.
Against this background, the object of the present invention is to improve a circuit arrangement as well as a method for generating a test voltage of the type mentioned in the introduction to the effect that these are suitable for generating a test voltage that is as interference-free as possible and a test current that is as interference-free as possible and for use in a test instrument that is as compact as possible and capable of highly precise loss-factor measurement.